Russian forces are closing in on Ukraine's second-biggest power plant at Vuhlehirska, 50 kilometres north-east of Donetsk, British military intelligence said on Thursday.
"Russia is prioritising the capture of critical national infrastructure, such as power plants," Britain's defence ministry said in a regular bulletin.
The ministry said Russia was probably trying to break through at Vuhlehirska, as part of efforts to regain momentum on the southern pincer of its advance towards the key cities of Kramatorsk and Sloviansk.
On the battlefront, the Ukrainian military has reported heavy and sometimes deadly Russian shelling amid what it said were largely failed attempts by Russian troops to advance.
Ukrainian forces have said they destroyed 17 vehicles, some of them armoured, and killed more than 100 Russian soldiers in the south and east.
The Russian-installed administration in the partially occupied Ukrainian region of Zaporizhzhia said Ukraine conducted a drone strike on a nuclear power station there, but that the reactor was not damaged.
Several blasts were heard in the Russian-controlled southern region of Kherson overnight and into Thursday, Russian news agency Tass reported.
Russia's invasion has killed thousands, displaced millions and flattened cities, particularly in Russian-speaking areas in eastern and south-eastern Ukraine.
The conflict has also raised global energy and food prices and increased fears of famine in poorer countries because Ukraine and Russia are both major grain producers.
The US estimates that Russian casualties in Ukraine so far have reached about 15,000 killed and perhaps 45,000 wounded, CIA director William Burns said on Wednesday.
Russia classifies military deaths as state secrets even in times of peace and has not updated its official casualty figures frequently during the war.
The US, which said on Tuesday that it saw signs Russia was preparing to formally annex territory it has seized in Ukraine, promised that it would oppose annexation.
"Again, we've been clear that annexation by force would be a gross violation of the UN Charter, and we would not allow it to go unchallenged. We would not allow it to go unpunished," State Department spokesman Ned Price said on Wednesday.
Russia annexed Crimea from Ukraine in 2014 and supports Russian-speaking breakaway entities – the Donetsk and Luhansk People's Republics (DPR and LPR) – in those provinces, together known as the Donbas.
Russian Foreign Minister Sergey Lavrov is the most senior figure to speak openly about the country's war goals in territorial terms, nearly five months after Russian President Vladimir Putin ordered the February 24 invasion while denying that Russia intended to occupy its neighbour.
Mr Lavrov told RIA Novosti that geographical realities had changed since Russian and Ukrainian negotiators held peace talks in Turkey in late March that failed to produce a breakthrough.
"Now the geography is different, it's far from being just the DPR and LPR, it's also Kherson and Zaporizhzhia regions and a number of other territories," he said, referring to territories well beyond the Donbas that Russian forces have wholly or partly seized.